When looking at the rules and supplements to Ars Magica, one
becomes aware of two great truths. The first is that the authors
have put together an incredibly good role playing game that
captures perfectly the feel of medieval magic, especially as seen
through our 20th-century lenses. The second is that the authors
have no idea who "Hermes" is, in the context of medieval magic.
They seem to think that what is referred to as the "Hermetic
tradition" descends from the worship of the Greek god Hermes
through the cult of his Roman eidolon Mercury to the post-Roman
survivals of knowledge in the Middle Ages.
This is incorrect. The "Hermetic tradition" of Western magic,
which is one of the most powerful forces in the "underground
stream" of Western culture, has less to do with the Roman god
Mercury than it does with hermetic sealing.

The "Hermes" referred to is Hermes Trismesgistos "Hermes the
Thrice- Great", who was conflated with the Egyptian god Thoth.
When the Greeks came to Egypt, they were incredibly impressed by
the ancient wisdom of the Egyptian priests. So impressed, that
they immediately plastered their gods' names all over the older
Egyptian ones in the grand old syncretic Greek tradition.
Hence Amon became Zeus-Amon and Thoth, god of letters and
sciences, became Thoth-Hermes (since Hermes invented the Greek
alphabet, don't you know).

Here is where Game Truth and Historical Truth diverge. In Game
Truth (where diseases are caused by an imbalance of humors and the
sun goes around the Earth) Hermes Trismesgistos was a very
powerful ancient mage. He was _not_ a god. No reputable medieval
magus believed in polytheism -- most, like John Dee, were devout
(if goofy) Christians. In his writings, collectively called the
Corpus Hermeticorum, Hermes describes himself as "Philosopher,
Priest, and King". Hence, he was human. An incredibly powerful
sorceror, to be sure, but not a god. His exploits included
building the Pyramids, designing the Hanging Gardens of Babylon,
teaching Pythagoras, and generally doing everything worth doing in
antiquity. He also invented the Egyptian alphabet and while doing
that and building the odd pyramid, became the inspiration for the
legends of Thoth. After his death (or occultation) he was
worshipped as a god, much as Alexander the Great was. Speaking of
Alexander the Great, it was he who discovered the Emerald Tablet
(on which was written the whole knowledge of magic in about two
paragraphs -- apparently it wasn't what he said, it was how he
said it) clenched in the mummified hands of Hermes Trismesgistos
him self. Other legends say he discovered it in the hands of the
ancient magician Apollonius of Tyana, but since Apollonius as born
four hundred years after Alexander died, most scholars doubt this
theory. Still other legends say the Tablet was discovered by
Sarah, wife of Abraham, which would make Hermes dead before he
built the Hanging Gardens. Anyway, the knowledge on the Tablet and
in Hermes T's other writings (17 or so known books plus
commentary) was what was called "Hermetic Science".

In Real History, the whole thing was faked up about 200 AD by the
Gnostic community in Alexandria who were big with the alchemists
who lived around there and then.

Either way, the Corpus survived in Greek libraries and later in
the Arab world. It was, however, lost in the West except for the
hints and allusions that bled through from Arabic contacts. The
itinerant occultist adept al-Farabi (890?-954) is described as
"Hermetic", and it is likely that the alchemical writings of Geber
(721-766), Rhazes (850-924) and Avicenna (980-1036) draw on the
_Corpus_ to some extent. The Arab alchemistic writings began to
filter into Europe following the Papacy of Sylvester II (999-1003)
and were eventually disseminated such that the legend of Hermes
Trismesgistos achieved a certain degree of recognition. The actual
Corpus did not become available to the West until 1460, when the
documents salvaged from Constantinople surfaced in Florence. Their
translation in 1471, by Marsilio Ficino, set off the great
explosion of Renaissance magic personified by Dee, Trithemius,
Agrippa, and Paracelsus.

This then, in a nutshell, is the "Hermetic Tradition": either the
Game Truth (2500 BC Hermes T builds Pyramids, is Thoth, lives
until c 550 BC when he finished off the Hanging Gardens, trained
Pythagoras, and died in a cave clutching his Emerald Tablet.
Discovered by Alexander, put in Library, alchemists study it,
knowledge lost with Fall of Rome, rediscovered in Dark Ages by
Vergil/Bonisagus/Aethelstan, spread by Order to this day) or the
Real Truth (200 AD Heretical alchemists work out consistent
philosophy, ascribe it to mythical figure to get credibility,
knowledge survives in alchemical tradition in Arab lands, stored
in Byzantium, filters into Europe c900-1250 AD from Spain,
Crusades, Sicily, rediscovered in 1471, all hell breaks loose).

This is so much cooler that one wonders why anyone would drag
those stuffy old Roman priests into it at all. "Hermetic sealing"
by the way, comes from an alchemical practice named for Guess Who.